Particle-beam device of the raster type

ABSTRACT

A raster type particle-beam device has an electro-optical axis and defines a specimen locality on the axis. The device has a beam generator for issuing a particle beam along the axis toward the specimen locality, a condenser lens for focussing the beam onto the specimen locality, the lens being disposed ahead of the specimen locality and coaxial with the axis, a deflection system arranged about the axis intermediate the beam generator and the condenser lens, an imaging surface disposed beyond the specimen locality in coaxial relation to the axis for receiving the rays of the beam passing through the specimen, and a beam modifying structure for modifying the imaging properties of the beam, the beam modifying structure being disposed in the path of the beam intermediate the deflection system and the specimen locality.

United stateS Patent [111 3,869,611

Thon Mar. 4, 1975 PARTICLE-BEAM DEVICE OF THE Primary Examiner-James W. Lawrence i 1 UN RASTER TYPE Inventor: Friedrich Thon, Berlin, Germany Siemens Aktiengesellschaft, Berlin and Munich, Germany Filed: Aug. 28, 1970 Appl. No.: 67,922

Assignee:

Foreign Application Priority Data Sept. 19, 1969 Germany 1948270 References Cited UNITED STATES PATENTS 6/l968 Le Poole ..250/49.5 A 3/l97l Herrman 250/495 D Assistant Examiner-B. C. Anderson Attorney, Agent, or FirmHerbert L. Lerner [57] ABSTRACT A raster type particle-beam device has an electrooptical axis and defines a specimen locality on the axis. The device has a beam generator for issuing a particle beam along the axis toward the specimen locality, a condenser lens for focussing the beam onto the specimen locality, the lens being disposed ahead of the specimen locality and coaxial with the axis, adeflection system arranged about the axis intermediate the beam generator and the condenser lens, an imaging surface disposed beyond the specimen locality in coaxial relation to the axis for receiving the rays of the beam passing through the specimen, and a beam modifying structure for modifying the imaging properties of the beam, the beam modifying structure being disposed in the path of the beam intermediate the deflection system and the specimen locality.

8 Claims, 1 Drawing Figure PMENTED 41975 I nvenlar:

My invention relates to a raster particle-beam devicewherein the specimen is scanned by the impinging rays in a predetermined pattern or raster.

1n the usual electron microscopes in which the partiole-beam, especially an electron beam, is focussed by means of a condensor lens on a small spot on the specimen under investigation wherewith the irradiated spot undergoes no change in position, it is known to provide means in the beam path in the imaging portion for'influencing the beam path to achieve desired image qualities or characteristics. Preferably, such means are diaphragms or phase plates arranged in the rear focal plane of the objective lens. In this connection, diaphragms required for dark-field imaging and having a central collector or half diaphragms and other special types are provided. Of special interest'are the zone diaphragms according to Hoppe as described in the U.S. Pat. No. 3,213,277; these diaphragms serve to reduce or eliminate the distrubing influences of the aperture error of the objective lens and are designed as zone diaphragms with alternating ray passing and ray blocking ring-shaped zones of definite spacing. The zones are dimensioned so that the zone diaphragm permits only those rays of the imaging beam to pass to the final image plane which contribute to forming images having only one character contrast, namely, either only positive phase or only negative phase. This zone diaphragm takes the facts into account that various phase displacements are imparted to the rays in the objective lens as well as in the specimen which lead to an undesired influencing of the quality of the final image. I

It has been proposed to provide a phase plate in the rear focal plane of the objective lens which influences or controls the phases of the rays of the beam such that they fulfill the phase conditions in the final image plane required for the desired image in spite of the phase displacements caused by passing through the specimen and the objective lens. Because a blocking of rays occur with the zone diaphragm of Hoppe which do not fulfill the requiredphase conditions, a correction of the phase errors is achieved by means of a phase plate in the beam path.

All these devices and others not mentioned above are placed in the imaging portion or more precisely behind the specimen in beam direction in the conventional electron microscopes.

The invention is not concerned withthis kind of particle-beam device. Instead, the invention relates to a raster particle-beam device such as a raster particlebeam microscope wherein the specimen is scanned by the impinging rays in a raster. A raster microscope of this type, which is described in the form of an electron raster microscope by von Ardenne in the Zeitschrift fur Physik, Volume 109, 1938, pages 553 to 572 as well as many more recent patents as, for example, the US. Pat. No. 3,389,252 or the British Pat. No. 1,128,107, contains a beam generator and sequentially, in beam direction, a deflection system for the beam, a condensor lens assembly for focussing the beam on a specimen whose image is to be formed and a registration or image plane in which the image signal of the specimen is'received.

A photo arrangement can be provided in this plane. Usually, a detector for particles and, if required, for light quanta is used.

- From FIG. 4 of the mentioned work of von Ardenne, it is known to provide special'means for dark-field operation in the beam path of such types of raster electron microscopes.

A raster particle-beam microscope constructed in this fashion and provided with a special means of this type is the subject matter of the present invention.

It is an object of my invention to provide a raster particle-beam device, such as a raster microscope, adapted to permit modifying the imaging properties of the beam.

It is another object of my invention to provide a raster particle-beam microscope wherein the special conditions pertaining to such microscopes are considered in applying the varied means for obtaining desired image qualities.

According to a feature of the invention, means for modifying the imaging properties of the beam are placed in the beam path intermediate the deflection system and the specimen locality.

The invention is based on the facts that the radiation aperture of a conventional through-radiation particlebeam microscope, that is one wherein the. specimen is not raster scanned, is generally small compared to the imaging aperture. This situation exists because the cross section of the imaging bundle of rays is determined by the objectiv aperture diaphragm and the imaging aperture is determined by the radius of this diaphragm and its spacing from the lens. In a raster particle-beam microscope just the opposite relations are presented wherein the. radiation aperture is large while the imaging aperture given by the diameter of the detector input diaphragm and the spacing of the specimen from the latter is small. If it is desired to install the modifying means such as diaphragms for modifying the image in a determined way into a raster microscope having the same operational arrangement as in the usual conventional through-beam microscopes, then the modifying means is arranged, according to the invention, between the deflection system and the specimen, that is in the irradiating portion of the microscope.

This applies also to the special situation given by Crewe in the US. Pat. No. 3,191,028 wherein the specimen is arranged in the field of the objective lens so that the last condensor lens is formed by the objective prefield. With this special raster microscope, the remainder of the objective lens field functions as a proper imaging lens, so that the rays scattered in the specimen also arrive at the detector with a larger exit aperture. To achieve the required contrast, a contrast'diaphragm is provided in front of the detector in beam direction,

the diaphragm serving to block individual rays. Also.

with this special raster microscope, which has its advantages in a high intensity of the beam at the detector input and in a smaller irradiating spot on the specimen, an additional beam modifying means in the form of a zone diaphragm, phase plate or the likeadvantageously is installed according to the invention in beam direction ahead of the specimen.

According to the preferred embodiment of the invention, the beam modifying means is arranged in the beam path between the deflection system and the condensor lens. Preferably, the modifying means is placed in the entrance diaphragm plane of the condensor lens. This special selection of location for the modifying means is taken because it desired to modify those rays of the beam which in the specimen plane run parallel.

According to the same principle, a preferred embodiment of a microscope of the invention has two deflection systems arranged about the electro-optical axis of the microscope in beam direction and spaced axially of each other. The first deflection system deflects the beam away from the electro-optical axis and the second deflection system deflects the beam toward the axis so as to intersect the latter approximately in the entrance diaphragm plane of the condensor lens. With another point of intersection of the deflected beam with the axis, different radiation directions are obtained and thereby different contrast conditions in dependence upon the distance of the intersection point from the specimen point under observation.

As already mentioned, the modifying means could comprise a diaphragm for blocking certain rays of the beam such as dark-field diaphragm with central receiver or a dark-field half diaphragm.

The application of dark-field technology to raster microscopes by means of using a centrally through bored detector is by itself known from FIG. 4 of the above referred to work of. von Ardenne. This known arrangement requires however, that the radiation aperture is small compared to the angle at which the electrons scattered in the specimen pass from this aperture. For

high resolving dark-field microscopy, there is in this regard desired that the radiation aperture is approximately the same as this scattering angle. The advantage of the device according to theinvention of a dark-field diaphragm in the radiation portion as compared to the known use of the diaphragm in the imaging portion is manifest by the suitability of the device according to the invention for high resolution investigations.

In addition, the diaphragm of the l-loppe type positioned in the radiation portion of a raster microscope according to the invention can be so dimensioned that only such rays of the beam reach the specimen which are imaged on the image receiver with only'one character of contrast after passing through the condensor lens and the specimen locality, namely either only with positive contrast or only with negative contrast. Because, with the usual arrangement of Hoppe-type zone diaphragms in the imaging portion of the microscope, those rays of the beam are blocked which are already so modified that they do not have the proper phase positions for contributing to the formation of an image at the final image plane, there is provided by a raster microscope according to the invention a pre-selection of the rays before they are modified or affected by the condensor lens and the specimen.

The foregoing applies to the introduction of a phase .plate that can be located at substantially the position of the diaphragm thisembodiment of the invention configuration has the features that the modifying means includes a phase plate that pre-affects a portion of the rays of said beam so that these rays meet all the phase conditions for forming an image after passing through the condensor lens and being affected by the specimen.

The invention will now be described with reference to the accompanying drawing. wherein the various members of a raster microscope according to the invention are shown disposed along the electro-optical axis 1 of the microscope.

The microscope is equipped with a beam generator of the usual construction having a point cathode 2, a

Wehnelt cylinder 3- and the anode 4. For example, the configuration of the beam generator'describedin German Pat. No. 1,031,447 can be applied. Also, a field emission cathode can be used.

In beam direction after the beam generator there are disposed deflection systems arranged in two stages one behind the other. The systems comprise two pairs of electrostatic deflection plates 5, 5' and 6, 6' which are supplied by saw tooth generators not illustrated. Additional deflection systems also not illustrated serve to defect the beam in a plane perpendicular to the plane of the drawing. The application of this type of deflection system in electrostatic configuration with raster microscopes is described in British Pat. No. 1,128,107.

It is also possible to provide electromagnetic deflection.

systems as described in U.S. Pat. No. 3,389,252. Finally, combined electrostatic and electromagnetic deflection systems are known from German Printed Pat. application No. 1,088,628.

In these combined systems the pole plates of the magnetic deflection system serve simultaneously as electrostatic deflection plates, so that the deflection in twodirections perpendicular to each other is achieved by means of a substantially unitary member.

As a condensor lens arrangement, there is a magnetic pole shoe lens disposed in the region ofthe beam path. A specimen 8 is situated in the back focal plane of the lens 7. The'specimen is preferably arranged in an adjustable specimen stage or in a diaphragm holder as de-' scribed for example in German Pat. No. 951,882.

The image is registered in the final image plane 9 by means of a suitable detector. Only those rays which are let through by thedetector entrance diaphragm 10 are included to form the image.

I The beam path delineated by the dashed line 11 results when the deflection systems 5, 5' and 6, 6 are deenergized or when the voltage applied to these systems passes through its zero value. The deflection systems are so arranged and the apparatus so dimensioned that the beam is deflectedoutwardly and inwardly by the deflection system in such a way that axis 13 of the inwardly deflected beam intersects the electro-optical axis 1 at the point 14 lying in the entrance-diaphragm plane of the condensor lens 7. The selection of this intersection point affords in connection with the position of the yet tob'e described diaphragm 15 in the same plane, the advantage that those rays, which run parallel to each other in the plane of the specimen 8, are influenced by the diaphragm 15 in accordance with beam path 16 bent out as shown when deflection systems 5, 5 and 6, 6' are energized.

To achieve desired imaging characteristics, a Hoppe type'zone diaphragm 15 is used. As indicated in the.

' constitutes an accurate imaging lens. The diaphragm 17 is disposed in the imaging portion behind the specimen in contrast to the diaphragm 15 arranged according to the invention in the radiation portion of the microscope. Diaphragm 17 also serves to ensure the contrast in the registration plane 9 by blocking a portion of the rays. Basically, condensor lenses can also be used in front of the deflection systems.

It will be understood by those skilled in the art that modifying means other than those herein specifically described can be applied to a raster particle beam device according to the invention for obtaining desired imaging qualities without departing from the essential features of the invention as set forth in the claims annexed hereto.

I claim:

1. A raster type charged particle-beam device having an electro-optical axis and defining a specimen locality on said axis, said device comprising beam generating means for issuing a beam of charged particles along said axis toward said specimen locality, a condensor lens for focussing said beam onto the specimen locality, said lens being disposed ahead of the specimen locality and coaxial with said axis, deflection means for deflecting the beam back and forth in raster fashion to raster scan a specimen placeable at the specimen locality, said deflection means arranged about said axis intermediate said beam generating means and said condensor lens, image receiving means disposed beyond the specimen locality in coaxial relation to said axis for receiving the rays of said beam passing through the specimen, and beam modifying means for modifying the imaging properties of the beam by cancelling a portion of the beam or modifying the phase of the rays thereof, said modifying means being disposed in the path of said beam in a transverse section of the beam and being disposed intermediate said deflection means and the specimen locality.

2. In a device according to claim 1, said modifying means being disposed in the path of said beam interme diate said deflection means and said condensor lens.

3. A raster type charged particle-beam device having an electro-optical axis and defining a specimen locality on said axis, said device comprising beam generating means for issuing a beam of charged particles along said axis toward said specimen locality, a condensor lens for focussing said beam onto the specimen locality, said lens being disposed ahead of the specimen locality and coaxial with said axis, deflection means for deflecting the beam back and forthin raster fashion to raster scan a specimen placeable at the specimen locality, said deflection means arranged about said axis intermediate said beam generating means and said condensor len's, image receiving means disposed beyond the specimen locality in coaxial relation to said axis for receiving the rays of said beam passing through the specimen, and beam modifying means for modifying the imaging properties of the beam by cancelling a portion of the beam or modifying the phase of'the rays thereof, said modifying means being disposed intermediate said deflection means and said condensor lens and being in the path of the beam so as to be in a transverse section thereof, said deflection means comprising first and second deflection systems arranged about said axis, said second deflection system being disposed axially of said first deflection system in beam direction toward said specimen locality, and first and second electric supply means connected to said first and second deflection systems respectively for energizing the same, whereby said first deflection system deflects said particle beam away from said axis and said second deflection system deflects said beam toward said axis so as to intersect the latter approximately at the locality of said modifying means.

4. In a device according to claim 3, said modifying means being disposed in the path of said beam. in the entrance diaphragm plane of said condensor lens.

5. In a device according to claim 1, said modifying means comprising a diaphragm disposed in the path of said beam and arranged between said deflection system and said condensor lens for blocking selected rays of said beam. r

6. In a device according to claim 5, said diaphragm being a dark-field diaphragm.

7. In a device according to claim 5, said diaphragm having mutually adjacent zones ray blocking and ray passing respectively, said zones being arranged so that only such rays reach the specimen-which are imaged on said irriage receiving means with only one character of contrast after passing through said condensor lens and the specimen locality, namely either only with positive contrast or only with negative contrast.

8. In a device according to claim 1, said modifying means comprising a phase plate disposed in the path of said beam for modifying the respective phases of a portion of the rays of said beam, said plate being arranged between said deflection means and said condensor lens. l l= 

1. A raster type charged particle-beam device having an electrooptical axis and defining a specimen locality on said axis, said device comprising beam generating means for issuing a beam of charged particles along said axis toward said specimen locality, a condensor lens for focussing said beam onto the specimen locality, said lens being disposeD ahead of the specimen locality and coaxial with said axis, deflection means for deflecting the beam back and forth in raster fashion to raster scan a specimen placeable at the specimen locality, said deflection means arranged about said axis intermediate said beam generating means and said condensor lens, image receiving means disposed beyond the specimen locality in coaxial relation to said axis for receiving the rays of said beam passing through the specimen, and beam modifying means for modifying the imaging properties of the beam by cancelling a portion of the beam or modifying the phase of the rays thereof, said modifying means being disposed in the path of said beam in a transverse section of the beam and being disposed intermediate said deflection means and the specimen locality.
 2. In a device according to claim 1, said modifying means being disposed in the path of said beam intermediate said deflection means and said condensor lens.
 3. A raster type charged particle-beam device having an electro-optical axis and defining a specimen locality on said axis, said device comprising beam generating means for issuing a beam of charged particles along said axis toward said specimen locality, a condensor lens for focussing said beam onto the specimen locality, said lens being disposed ahead of the specimen locality and coaxial with said axis, deflection means for deflecting the beam back and forth in raster fashion to raster scan a specimen placeable at the specimen locality, said deflection means arranged about said axis intermediate said beam generating means and said condensor lens, image receiving means disposed beyond the specimen locality in coaxial relation to said axis for receiving the rays of said beam passing through the specimen, and beam modifying means for modifying the imaging properties of the beam by cancelling a portion of the beam or modifying the phase of the rays thereof, said modifying means being disposed intermediate said deflection means and said condensor lens and being in the path of the beam so as to be in a transverse section thereof, said deflection means comprising first and second deflection systems arranged about said axis, said second deflection system being disposed axially of said first deflection system in beam direction toward said specimen locality, and first and second electric supply means connected to said first and second deflection systems respectively for energizing the same, whereby said first deflection system deflects said particle beam away from said axis and said second deflection system deflects said beam toward said axis so as to intersect the latter approximately at the locality of said modifying means.
 4. In a device according to claim 3, said modifying means being disposed in the path of said beam in the entrance diaphragm plane of said condensor lens.
 5. In a device according to claim 1, said modifying means comprising a diaphragm disposed in the path of said beam and arranged between said deflection system and said condensor lens for blocking selected rays of said beam.
 6. In a device according to claim 5, said diaphragm being a dark-field diaphragm.
 7. In a device according to claim 5, said diaphragm having mutually adjacent zones ray blocking and ray passing respectively, said zones being arranged so that only such rays reach the specimen which are imaged on said image receiving means with only one character of contrast after passing through said condensor lens and the specimen locality, namely either only with positive contrast or only with negative contrast.
 8. In a device according to claim 1, said modifying means comprising a phase plate disposed in the path of said beam for modifying the respective phases of a portion of the rays of said beam, said plate being arranged between said deflection means and said condensor lens. 